1. Field of the Invention
The present invention relates to an optical disc apparatus having a wobble signal detection circuit, an information recording method, apparatus and method of adjusting an optical disc apparatus, a method of fabricating an optical disc apparatus, and apparatus and method of adjusting a phase shift amount.
2. Description of the Related Art
For optical discs including DVD+R and DVD+RW, an optical disc having an information track wobbled at a constant time period, the information track being spirally formed on an optical disc substrate in advance in a manner of phase shift keying, is used.
In a light receiving element receiving light reflected from such an optical disc including DVD+R and DVD+RW, for example, in a wobble signal detection circuit used to generate ADIP (Address In Pre-groove) information or the like by detecting and generating a wobble signal based on a received light signal of a four-divided light receiving element, a circuit as illustrated in FIG. 20 is used. For the illustrated wobble signal detection circuit, only a first half portion, that is, a portion used until a digital wobble signal and an analog wobble signal are generated based on voltage signals VA through VD obtained from respective light receiving element areas of a four-divided light receiving element is illustrated.
First, the individual voltage signals VA through VD are sample-held by respective sample-hold circuit (S/H) 100, and after adders 101 add VA and VB to VC and VD, respectively, multipliers 102 reduce the results to half, that is, the respective multipliers 102 compute 0.5(VA+VC) and 0.5(VB+VD). Then, a balance AGC circuit 103 balances amplitudes between 0.5(VA+VC) and 0.5(VB+VD), and a subtracter 104 computes 0.5(VA+VC)−0.5(VB+VD) and supplies the computation result to a filter circuit 105. The filter circuit 105 includes a BPF path for digital wobble signals provided by a band-pass filter (BPF) 106 and an HPF+LPF path for analog wobble signals provided by a high-pass filter (HPF) 107 and a low-pass filter (LPF) 108. A digitizer 109, for example, using a comparator, is provided at an output side of BPF 106.
Here, a difference of a cutoff frequency fc in LPF 108 of the HPF+LPF path becomes problematic. It is hard to make the cutoff frequency fc in LPF 108 accurate, and in particular, the difference between circuits is noticeable in mass production. If this cutoff frequency fc is not uniform, as illustrated in FIG. 21, the phase of the analog wobble signal also deviates from an ideal phase (solid line) depending on high and low levels. When information is recorded on an optical disc such as DVD+RW, the recording start position is set on the basis of an analog wobble signal. Thus, if the phase of the analog wobble signal deviates, the recording start position is also misplaced from the optimal position, as illustrated in FIG. 22(b).
In order to overcome this problem, for example, according to Patent Reference 1 (Japanese Laid-Open Patent Application No. 2000-173055), a method is presented. In the method, when information is overwritten on an already recorded disc, data reproduced from the recorded portion thereof are used as a reference signal for a clock generator. Also, in a newly information-recorded area, the clock phase is adapted by switching into a clock generation signal recorded in the recorded portion in advance, and the recording start position is set to the optimal value.
Apart from the method in Patent Reference 1, there is a method of adjusting the recording start position to the optimal recording position by using a disc whose 14T sync frame part is provided with a 6T mirror part (portion where lighting is impossible and the reflection rate is high). In this adjustment method, each one ECC (one record block of DVD) is recorded on a mirror disc, and after the recording, the disc is read to observe RF signal waveform (see FIG. 23). Then, the optimal recording position is determined based on a phase difference between the center of the 14T portion of the RF signal and the center of the mirror portion (if the 14T center matches the mirror portion center, the recording start position is optimal). After that, the recording start position is adjusted by setting a recording start position setup parameter (which is referred to as timeset hereinafter, see FIG. 24) P1, P2 or P3 corresponding to the phase difference. Thus, even if the wobble signal is phase lagged, it is possible to adjust the recording start position to the optimal recording position through adjustment of timeset.
However, Patent Reference 1 is applicable to only an optical disc having a recorded portion. Also, although Patent Reference 1 can be successfully applied to an optical disc having a recorded portion where the recording has been conducted at a correct recording start position, the method of Patent Reference 1 can start recording from an incorrect recording start position for an optical disc having a recorded portion where the recording has not been conducted at a correct recording position.
Also, in the latter case, since the adjustment method requires to use a special disc, the cost becomes higher. Additionally, since information must be actually recorded in a disc and then be actually reproduced from the disc to check the status of an RF signal, it takes longer time to adjust the recording start position.
Additionally, in terms of the interior of a filter circuit, for example, if the phase relation between an analog wobble signal and a digital wobble signal is not uniform due to the nonuniformity of the cutoff frequency fc of LPF, reasonable ADIP information cannot be obtained because of failure of multiplication. As a result, address information and synchronization signals may be adversely affected.